To add a harmonic to an original signal, without mixing loopback noises even in the case of decimation by adding a harmonic to a digital audio signal and passing a resulting signal through a band-pass filter(BPF).

A digital audio signal whose sampling frequency is fs is given to an oversampling circuit 2, where the signal is re-sampled into a digital audio signal whose sampling frequency is 2fs. A 1st low-pass filter(LPF) 3 eliminates a signal component whose frequency is fs/2 or over. The digital audio signal is outputted simultaneously to a nonlinear processing circuit 4 and to a delay section 5. The digital audio signal received by the nonlinear processing circuit 4 is added to a harmonic, and part mixed with loopback noise is eliminated from the resulting signal given to a BPF 6. An output from the BPF 6 and an output from the delay section 5 are added by an adder 7. A decimation circuit 8 applies decimation the digital audio signal.